The present invention relates to a driver for a stepping motor and, more particularly, to a driver circuit which is effective for use in a floppy disk apparatus.
In a floppy disk apparatus a stepping motor is used to move (seek) a head in the radial direction of the disk. There have been various types of stepping motor drivers, such as the three-phase Variable Reluctance (VR) type, four-phase Permanent Magnet (PM) type, four-phase hybrid type and five-phase hybrid type. The four-phase hybrid type is generally used.
The four-phase hybrid type stepping motor has four or two windings. The former type (four windings) stepping motor is rotationally driven by suitably selecting the winding direction and arrangement of the respective windings to make the magnetomotive forces (or magnetic field directions) of the respective windings different by 90 degrees and to sequentially vary the magnetizing order of the windings. In the latter type (two windings), the four magnetic field directions having a phase difference of 90 degrees are equivalently realized by varying the directions of currents flowing through the two windings which are arranged in electrically spaced positions of 90 degrees.
FIG. 1 shows an example of a driver circuit for use with two windings W.sub.A and W.sub.B according to the prior art. The directions of the currents flowing through the windings W.sub.A and W.sub.B are varied by controlling switching transistors TR.sub.1 to TR.sub.8. The driver circuit shown in FIG. 1 is usually called the "H-bridge".
In the floppy disk device, the stepping motor causes the magnetic head to seek the radial position of the disk in response to a step pulse and determines the seeking direction in response to a direction signal. As the magnetizing method of the windings there have been utilized one-phase magnetization for magnetizing one winding and two-phase magnetization for concurrently magnetizing two windings. The one-phase magnetization may obtain higher accuracy in stopping position whereas the two-phase magnetization may obtain larger drive torque. In order to attain the larger drive torque, the two-phase magnetization is usually used.
The floppy disk device has two states--the seeking state in which the magnetic head is caused to seek the radial position of the disk and the holding state in which the magnetic head is held in a predetermined position. It is the usual practice to feed a larger current for the seeking operation and a smaller current for the holding operation. This is because the torques necessary are different for the seeking and holding operations (that is, the larger torque is required for the former), whereby the calorific power is desirably dropped so as to avoid the thermal expansion of the Mylar used as the base of the disk medium, and the load upon the host power supply is desirably reduced.
The switching of the currents for the seeking and holding operations is effected by adopting either the two-power supply method or the resistor insertion method in the prior art. FIG. 1 shows the two-power supply method, in which the windings W.sub.A and V.sub.B are supplied with power of +12V, when a switching transistor TR.sub.0 is "ON", and at +5V, when the same is "OFF", via a diode D.sub.1 through transistors TR.sub.1 -TR.sub.8.
FIG. 2 is a circuit diagram showing the resistor insertion method, in which a voltage of +5 V is applied to the windings when the switching transistor TR.sub.0 is "ON" whereas a lower, voltage that by the voltage drop of a resistor R.sub.1, than +5V is applied to the windings when the same is "OFF". FIG. 3 is a circuit diagram showing another example, in which resistors R.sub.2 and R.sub.3 are connected in series with the windings W.sub.A and W.sub.B and are short-circuited, if necessary, by switches SW.sub.1 and SW.sub.2.
The circuit of FIG. 2 is utilized because the number of its parts is fewer than that of the circuit of FIG. 3. In the resistor insertion method, however, the voltage to be applied to the H-bridge (especially, the transistors TR.sub.1, TR.sub.3, TR.sub.5 and TR.sub.7) will drop in accordance with the voltage drop due to the resistor R.sub.1 which will make the bias of the PNP and NPN series transistors unreliable and insufficient and cause oscillating phenomena. With a view to compensating the delay of the base currents of the transistors, therefore, a capacitor C of high capacitance is inserted, as shown.
However, the aforementioned two-power supply method requires two kinds of power supplies which complicates the device and prevents size reduction. In the resistor insertion type method, on the other hand, the resistor inserted causes unnecessary power consumption and calorific power and prevents integration of the circuit. The requirement of a large value such as an electrolytic capacitor makes it impossible to realize the integration of the circuit and causes problems as to reliability, i.e., working life.